Formation of Planetary Nebulae Home The Not-so-Ordinary Life of a Typical Star Scattered like diamond chips across the Cosmos, stars look deceptively serene to earthbound observers. However, nothing could be farther from true because each one is a creature of unimaginable violence. Constant and unchanging, the nighttime stellar canopy appears essentially the same to us as it did to the ancient Greeks. But, this perception is misleading because, like people, even stars follow a cycle of birth and death. A star, like our Sun, begins its life as a vast cloud of gas and dust drifting among the apparently empty spaces between the stars. These clouds of sparse material are truly immense and often span hundreds or thousands of light years in all directions. Even though they contain fewer atoms than the best vacuum on Earth, the total amount of material in an interstellar cloud, also known as its mass, is truly astronomical. Most of the stuff in one of these clouds is hydrogen, the simplest and most abundant element in the Universe, but they also contain other, more complex, elements, too. For example, the cloud of dust and gas that formed our Sun must have included material from a previous generation star that blew itself apart.

We suspect this because our Sun, and all of its planets including Earth, contain heavy elements that could only have been forged in the heart of a larger, long gone star that went supernova. Stars, like factories, create complex elements from hydrogen and release radiation as a by-product of their creation. The life cycle of a typical star like our Sun. Illustration credit: Wikipedia For billions of years interstellar clouds wander more or less aimlessly between their starry relatives until they receive a gravitational nudge, possibly from a nearby exploding star or the close passing of another galaxy. Significantly, once the cloud is pushed, events take on a life of their own and the material begins to contract inward under its own massive weight. As the dust particles and gas molecules move toward the cloud's center, they inevitably begin bumping into each other. Like rubbing your hands together, this generates heat. Stellar womb- am artist's impression of the central region inside an interstellar cloud where gravity is forging new stars. Illustration credit: M. Kornmesser/ESO Over time, these molecular collisions became more frequent. Eventually, the core of the cloud becomes so squeezed by gravity that pressures exceeded billions of atmospheres and temperatures reach over 50 million degrees.

At this temperature, hydrogen nuclei collide with such speed, a thermonuclear reaction, known as fusion, is triggered. Fusion is the same process released when a hydrogen bomb is exploded. However, the amount of energy produced by a star is billions of times greater than any man-made nuclear weapon. When fusion commences, hydrogen is converted into the next heaviest element (helium) and a tremendous amount of energy is released in the form of photons. As the photons rush from the core of the cloud, they push back on the inward pressure of gravity, which is still trying to compress the cloud, until an equilibrium is achieved and the cloud's collapse ceases. Eventually the photons escape into space as visible light and other forms of invisible radiation and the star is born. Thus, a star is a balance between the unrelenting inward pull of gravity and the ongoing thermonuclear explosion pushing back from its core. Phase One- Nothing lasts forever Although, most of a star's volume is comprised of hydrogen, hydrogen can only be fused into helium at its core and all stars eventually deplete their central store of hydrogen. Thus, the balance that creates a star is only temporary.

For example, although our Sun is supremely important to our existence, it is only an average star, about halfway through its 10 billion year life-cycle. Stars with less mass require less thermonuclear energy at their cores to counter balance the force of gravity, therefore they are cooler and shine for a longer period of time. Conversely, stars with greater mass than the Sun are hotter because they require more outward force to prevent gravity from squeezing them. Thus, larger stars are brighter and shine for a shorter period of time. Our local star has sufficient mass in its core to fuse 700 million tons of hydrogen into 695 million tons of helium each second for 10 billion years. In the process, 5 billion tons of matter is also converted into energy per second as described by Einstein's famous equation E = mc2. At this rate, the Sun has already converted about 100 Earth-masses of matter into energy. But eventually, all the hydrogen in its core will fused into helium.

When this happens, nuclear reactions will cease, the outward force of energy that has supported the star's weight will drop precipitously and the core will begin to compress under the force of gravity that has been patiently waiting all these years. Phase Two- In the Footsteps of Giants As the core collapses, the pressure and temperature within it will increase until the helium begins to fuse into the next heaviest elements, carbon and oxygen- two of the most important elements required for life as we know it. A huge amount of radiation will be released when this occurs - more than the amount released by the fusion of hydrogen. This further increases the pressure pushing back on gravity. The heat of the interior becomes so intense, a layer of hydrogen surrounding the core begins to fuse into helium. This also increases the amount of outward pressure and, combined, both these forces cause the star to grow enormous. Gravity eventually balances this expansion, but only after the star has swollen to the orbit of Venus or possibly even farther out. In the case of our Sun, the planet Mercury will be absorbed along with Venus and Earth may be engulfed or thrown from its current orbit in the process.

Since the star has the same amount of material, its heat will be spread over a larger surface area and thus it will be cooler, take on a reddish hue and become a star that is called Red Giant . Red Giant Sun - In about 5 billion years our Sun will swell and possibly consume the Earth. In this artist conception, the distant moon transits the swollen solar disk as seen from our then lifeless, barren planet. Illustration credit: Don Dixon Cosmographia Phase 3- The Long Good-bye As was the case with hydrogen, the amount of helium in the core will eventually become exhausted. When this happens, gravity will cause the surface of the star to suddenly shrink as it simultaneously exerts enormous pressure on the carbon and oxygen core. However, in a star like our Sun, core temperatures will never become sufficient to trigger the fusion of carbon and oxygen into the next heavier elements. It will only succeed in triggering fusion of the helium shell surrounding the core. When thermonuclear reactions within the shell of helium commences, the outward radiation pressure will exceed the inward pull of gravity and the surface of star will expand once again returning the star to its Red Giant size.

However, this time, the force of gravity will be insufficient to stop the expansion. So, the outer portions of the star will separate at up to 50 km/sec (over 100,000 miles/hour) and float away from the central region containing the star's core. The ejected gas will begin to visibly glow in harlequin colors as it is ionized by invisible ultraviolet radiation still being released from the star's hot core. As the star's outer surface material departs the core will be progressively exposed. When the core of a star is revealed in this manner, the star is called a white dwarf . These stars are fantastically dense compared to anything on Earth, weighing over a ton per teaspoonful. Over billions of years, the core will slowly cool, cease to release any radiation and become a black dwarf - the corpse of a Sun that once was. If the star had a family of planets, those that weren't devoured or ejected from their solar system during its Red Giant phase will freeze. The Dumbbell nebula (NGC 6853) The Universe is filled with spectacular cosmic displays, but few match the wonder of a planetary nebula. Photo credit: R.

Jay GaBany Cosmotography.com The First Planetary About 4,000 years ago an elderly, bloated giant red star, located toward the northern constellation of Vulpecula, gave its last gasp and shed its outer skin exposing its still pulsating heart. It wasn't until the mid-1700s that Charles Messier, the famous French comet hunter, noticed its distant faint fuzzy circular glow and placed it in his catalog to prevent him from mistaking it as a comet during future night sky expeditions. A few years later, the musician turned astronomer named William Herschel gave these objects their name because of their resemblance to planets. Telescopes back then lacked the color definition and clarity of even the most inexpensive instruments available today. So, both Messier and Herschel would most likely be astonished had they lived to enjoy our current view of this planetary nebula. Today, we know Messier's 27th catalog designation as the Dumbbell nebula. Located about 1,200 light years from Earth. At that distance, our sun would appear 100 times fainter than the nebula. We happen to view the Dumbbell along its equator.

If our line of site were more to its poles, our impression of the Dumbbell might be that of a ring. The Dumbbell is approximately half a light year in diameter but even at this great distance, the nebula appears quite large- equivalent to about half the diameter of the full Moon. Our galaxy contains over 200 billion stars but only about 3,000 are surrounded by a planetary nebula. Planetary nebulae are usually no larger than a light-year in diameter which is actually enormous if you consider a light-year is almost 6 trillion miles in length. Because a planetary's bubble of gas is constantly expanding, the Dumbbell will only remain visible for around 10,000 years like most other planetary nebulae. Compared to the life of a star, which stretches into billions of years, a planetary nebula only exists for the twinkling of an eye and that helps explain their rarity. Planetary nebulae come is a variety of shapes, no two are exactly identical and this has been a source of ongoing study by astronomers. Only about 20% of the known planetary nebulae have a spherical symmetry.

Most, like the Dumbbell nebula, are elliptical or bi-polar with elongated somewhat mirrored structures protruding on opposite sides. The reason why most planetary nebulae are not spherical is not well understood. Several theories have been considered so far. One of them suggests the strange shapes of planetary nebulae might be due to some centrifugal force resulting from the rotation of red giant stars. Another theory proposes the symmetry of the star's wind may be affected by a companion star. However, the most recent theories explaining the shapes of the nebulae attribute their appearance to magnetic fields from the parent star. Perhaps the best explanation for the structures seen within and surrounding the Dumbbell come from a 2008 investigation . The researchers proposed the existence of a jet spawned by an accretion disk, fed by material from a small stellar companion, surrounding the nebula's parent star. Over time, precession, similar to the wobble seen in a spinning top, causes the jet to slew in different directions. As the jet moves, it creates conical shaped structures that give the surrounding nebula its dumbbell appearance. It's also amazing what scientists can learn from studying the colors released by stars, emitted or reflected by nebula.

For example, the Dumbbell's teal coloration is released by oxygen atoms while the red hues are emitted by hydrogen when both are struck by the ultraviolet radiation sill being released from the core of the planetary's parent star. A New Beginning The creation of even ordinary stars requires the release of incredible energy, millions of times more powerful than every warhead and bomb in the world's combined nuclear arsenals. Even after its birth, the fury of a star can only be tamed by the relentless force of gravity. From our perspective, safely sequestered by great distance, the twinkling stars of the nighttime sky seem timeless, conjure thoughts of romance and appear dazzling to our eyes. But, as we have discussed, nothing could be farther from the truth. Yet, if it weren't for the life cycle of stars, our existence would be impossible. Like stellar factories, stars transmute the building blocks of everything that surrounds us and in their deaths release a precious cargo of stellar material which may eventually find its way into creatures such as we, ourselves. In this way, instead of an end, every planetary nebula signals a new beginning.

Also read: By studying other stars astronomers can predict what the - DEPARTMENT - 9 View Full Document By studying other stars, astronomers can predict what the rest of the Sun’s life will be like. About five billion years from now, the core of the Sun will shrink and become hotter. The surface temperature will fall. The higher temperature of the center will increase the rate of thermonuclear reactions. The outer regions of the Sun will expand approximately 35 million miles, about the distance to Mercury, which is the closest planet to the Sun. The Sun will then be a red giant star. Temperatures on the Earth will become too high for life to exist. Once the Sun has used up its thermonuclear energy as a red giant, it will begin to shrink. After it shrinks to the size of the Earth, it will become a white dwarf star. The Sun may throw off huge amounts of gases in violent eruptions called nova explosions as it changes from a red giant to a white dwarf. After billions of years as a white dwarf, the Sun will have used up all its fuel and will have lost its heat. Such a star is called a black dwarf. After the Sun has become a black dwarf, the Earth will be dark and cold.

If any atmosphere remains there , it will have frozen over the Earth’s surface. Câu 36: It can be inferred from the passage that the Sun ______. A. has been in existence for 10 billion years B. is approximately halfway through its life as a yellow dwarf C. will continue to be a yellow dwarf for another 10 billion years D. is rapidly changing in size and brightness Câu 37: What will probably be the first stage of change for the Sun to become a red giant? A. Its core will cool off and use less fuel. B. Its central part will grow smaller and hotter. C. It will throw off huge amounts of gases. D. Its surface will become hotter and shrink. Câu 38: When the Sun becomes a red giant, what will the atmosphere be like on the Earth? A. It will become too hot for life to exist. B. It will be enveloped in the expanding surface of the sun. C. It will freeze and become solid. D. It will be almost destroyed by nova explosions. Câu 39: When the Sun has used up its energy as a red giant, it will ______. A. stop to expand B.

cease to exist C. become smaller D. get frozen Câu 40: Large amounts of gases may be released from the Sun at the end of its life as a ______. A. white dwarf B. red giant C. yellow dwarf D. black dwarf Câu 41: As a white dwarf, the Sun will be ______. A. a cool and habitable planet B. the same size as the planet Mercury C. thousands of times smaller than it is today D. around 35 million miles in diameter Câu 42: The Sun will become a black dwarf when ______. A. the outer regions of the Sun expand B. it has used up all its fuel as a white dwarf C. the Sun moves nearer to the Earth D. the core of the Sun becomes hotter Câu 43: The word “ there ” in the last sentence of paragraph 4 refers to ______. A. our own planet B. the core of a black dwarf C. the outer surface of the Sun D. the planet Mercury Câu 44: This passage is intended to ______. This preview has intentionally blurred sections. Sign up to view the full version. This is the end of the preview.

Sign up to access the rest of the document. TERM Vietnam University of Commerce DEPARTMENT 9 - Fall 2014 B GIO DC V O TO CHNH THC ( thi c 05 trang) THI TUYN SINH I HC, CAO NG NM 2008 Mn th DeAnhDCt_M978 Chapter 20, Chapter Review SUMMARY Stars spend most of their lives on the main sequence, in the core-hydrogen burning phase of stellar evolution, stably fusing hydrogen into helium at their centers. Stars leave the main sequence when the hydrogen in their cores is exhausted. For the Sun, which is about halfway through its main-sequence lifetime, this stage will occur about 5 billion years from now. Low-mass stars evolve much more slowly than the Sun, and high-mass stars evolve much faster. When the central nuclear fires in the interior of a solar-mass star cease, the helium in the stars core is still too cool to fuse into anything heavier. With no internal energy source, the helium core is unable to support itself against its own gravity and begins to shrink.

The star at this stage is in the hydrogen-shell burning phase, in which the nonburning helium at the center is surrounded by a layer of burning hydrogen. The energy released by the contracting helium core heats the hydrogen-burning shell, greatly increasing the nuclear reaction rates there. As a result, the star becomes much brighter while the envelope expands and cools. A low-mass star like the Sun moves off the main sequence on the HR diagram first along the subgiant branch , then almost vertically up the red-giant branch . As the helium core contracts, it heats up. Eventually, the core of a star more than 0.25 times the mass of the Sun reaches the point at which helium begins to fuse into carbon. The net effect of the fusion reactions is that three helium nuclei (or alpha particles) combine to form a nucleus of carbon in the triple-alpha process . In a star like the Sun, conditions at the onset of helium burning are such that the electrons in the core have become degeneratethey can be thought of as tiny, hard spheres that once brought into contact, present stiff resistance to being compressed any further.

This electron degeneracy pressure makes the core unable to react to the new energy source, and helium burning begins violently in the helium flash . The flash expands the core and reduces the stars luminosity, sending it onto the horizontal branch of the HR diagram. The star now has a core of burning helium surrounded by a shell of burning hydrogen. As helium burns in the core, it forms an inner core of nonburning carbon. The carbon core shrinks and heats the overlying burning layers, and the star once again becomes a red giant. It reenters the red-giant region of the HR diagram along the asymptotic-giant branch, becoming an extremely luminous red-supergiant star. The core of a low-mass star never becomes hot enough to fuse carbon. Such a star continues to ascend the asymptotic-giant branch until its envelope is ejected into space as a planetary nebula . At that point the core becomes visible as a hot, faint, and extremely dense white dwarf. The planetary nebula diffuses into space, carrying helium and some carbon into the interstellar medium. The white dwarf cools and fades, eventually becoming a cold black dwarf.

Most white dwarfs are composed of carbon and oxygen, although stars in binary systems may give rise to helium white dwarfs, while more massive stars may become neon-oxygen white dwarfs. Evolutionary changes happen more rapidly for high-mass stars than for low-mass stars because larger mass results in higher central temperatures. High-mass stars do not experience a helium flash and attain central temperatures high enough to fuse carbon. They form heavier and heavier elements in their cores, at a more and more rapid pace, and eventually die explosively. The theory of stellar evolution can be tested by observing star clusters, all of whose stars formed at the same time. As time goes by, the most massive stars evolve off the main sequence first, then the intermediate-mass stars, and so on. At any instant, no stars with masses above the clusters main-sequence turnoff mass remain on the main sequence. Stars below this mass have not yet evolved into giants and so still lie on the main sequence. By comparing a particular clusters main-sequence turnoff mass with theoretical predictions, astronomers can measure the age of the cluster. Stars in binary systems can evolve quite differently from isolated stars because of interactions with their companions.

Each star is surrounded by a teardrop-shaped Roche lobe , which defines the region of space within which matter belongs to the star. As a star in a binary evolves into the giant phase it may overflow its Roche lobe, forming a mass-transfer binary as gas flows from the giant onto its companion. If both stars overflow their Roche lobes, a contact binary results. Stellar evolution in binaries can produce states not achievable in single stars. In a sufficiently wide binary, both stars evolve as though they were isolated. SELF-TEST: TRUE OR FALSE? 1. High-mass stars are conventionally taken to have masses greater than that of the Sun. HINT 2. All the red-dwarf stars that ever formed are still on the main sequence today. HINT 3. Once on the main sequence, gravity is no longer important in determining a stars internal structure. HINT 4. The Sun will get brighter as it begins to run out of fuel in its core. HINT 5. As a star evolves away from the main sequence, it gets larger. HINT 6. As a star evolves away from the main sequence, it gets hotter. HINT 7.

As a red giant, the Sun will have a core that is smaller than it was when the Sun was on the main sequence. HINT 8. When helium starts to fuse inside a solar-mass red giant, it does so slowly at first, and the rate of fusion increases gradually over many years. HINT 9. With the onset of helium fusion, a red giant gets brighter. HINT 10. A planetary nebula is the disk of matter around a star that will eventually form a planetary system. HINT 11. For a high-mass star, there is no helium flash. HINT 12. High-mass stars can fuse carbon and oxygen in their cores. HINT 13. A star cluster with an age of 500 million years will contain many O-type stars. HINT 14. In a binary-star system, it is never possible for the lower-mass star to be more evolved than the higher-mass companion. HINT 15. In a mass-transfer binary, one of the stars has filled its Roche lobe. HINT SELF-TEST: FILL IN THE BLANK 1.

A main-sequence star doesnt collapse because of the outward _____ produced by hot gases in the stellar interior. HINT 2. The Sun will leave the main sequence in about _____ years. HINT 3. While a star is on the main sequence _____ is slowly depleted and _____ builds up in the core. HINT 4. A temperature of at least _____ is needed to fuse helium. HINT 5. At the end of its main-sequence lifetime, a stars core starts to _____. HINT 6. When helium fuses, it produces _____ and releases _____. HINT 7. Just before helium fusion begins in the Sun, the cores outward pressure will be provided mainly by electron _____. HINT 8. As a star ascends the asymptotic-giant branch, its _____ core is shrinking in size and growing in mass. HINT 9. As a red supergiant the Sun will eventually become about ____ times its present size. HINT 10. The various stages of stellar evolution predicted by theory can be tested using observations of stars in _____. HINT 11.

By the time the envelope of a red supergiant is ejected, the core has shrunk to a diameter of about _____. HINT 12. A typical white dwarf has the following properties: about half a solar mass, fairly _____ surface temperature, small size, and _____ luminosity. HINT 13. As time goes by, the temperature and the luminosity of a white dwarf both _____. HINT 14. As a star cluster ages, the luminosity of the main-sequence turnoff _____. HINT 15. Whether being a member of a binary-star system will affect the evolution of a star depends largely on the _____ of the two stars in the binary. HINT REVIEW AND DISCUSSION Algorithmic versions of these questions are available in the Practice Problems Module of the Companion Website. The number of squares preceding each problem indicates the approximate level of difficulty. 1. The Sun will evolve off the main sequence when roughly 10 percent of its hydrogen has been fused into helium. Using the data given in Section 16.5 and Table 16.2, calculate the total amount of mass destroyed (that is, converted into energy) and the total energy released by the fusion of this amount of matter.

HINT 2. Use the radiusluminositytemperature relation to calculate the radius of a red supergiant with temperature 3000 K (half the solar value) and luminosity 10,000 solar luminosities. (Sec. 17.3) How many planets of our solar system would this star engulf? HINT 3. What would be the luminosity of the Sun if its surface temperature was 3000 K and its radius was (a) 1 A.U., (b) 5 A.U.? HINT 4. Use the radiusluminositytemperature relation to calculate the radius of a 12,000 K (twice the temperature of the Sun), 0.0004-solar-luminosity white dwarf. HINT 5. Use the graph in More Precisely 20-1 to estimate the factor by which CNO energy production outstrips protonproton energy production in a 10-solar-mass star with a central temperature of 25 million K.

What do you think the factor would be if the abundances of C, N, and O were just one-tenth the solar value? HINT 6. A main-sequence star at a distance of 20 pc is barely visible through a certain telescope. The star subsequently ascends the giant branch, during which time its temperature drops by a factor of three and its radius increases 100-fold. What is the new maximum distance at which the star would still be visible using the same telescope? HINT 7. A Sunlike star goes through a rapid luminosity change between stages 8 and 9, when the luminosity increases by about a factor of 100 in 105 years. On average, how rapidly does the stars absolute magnitude change, in magnitudes per year? Do you think this change would be noticeable in a distant star within a human lifetime? HINT 8. Calculate the average density of a red-giant core of 0.25 solar-mass and radius 15,000 km. Compare this with the average density of the giants envelope, if it has a 0.5 solar-mass and its radius is 0.5 A.U.

Compare each with the central density of the Sun. (Sec. 16.2) HINT 9. How long will it take the Suns planetary nebula, expanding at a speed of 50 km/s, to reach the orbit of Neptune? How long to reach the nearest star? HINT 10. What are the escape speed (in km/s) and surface gravity (relative to Earths gravity) of Sirius B? (See Table 20.2.) HINT 11. A 15-solar-mass blue supergiant with a surface temperature of 20,000 K becomes a red supergiant with the same total luminosity and a temperature of 4000 K. By what factor does its radius change? HINT 12. The radius of Betelgeuse varies by about 60 percent within a period of three years. If the stars surface temperature remains constant, by how much does its absolute magnitude change during this time? HINT 13. The Sun will reside on the main sequence for 1010 years.

If the luminosity of a main-sequence star is proportional to the fourth power of the stars mass, what mass star is just now leaving the main sequence in a cluster that formed (a) 400 million years ago, (b) 2 billion years ago? HINT 14. In roughly 5 billion years, the Sun will eject its envelope as a planetary nebula. Before then, suppose it loses 20 percent of its mass on the giant branch. (a) If Jupiter stays in a circular orbit while this mass is being lost and its angular momentum stays constant, what will be the planets orbital radius and period when the Suns mass has fallen to 0.8 solar masses? (b) When the planetary nebula is formed, the Sun loses a further 0.3 solar masses rapidly enough that we can regard the loss as immediate. Jupiters instantaneous velocity is unchanged, but the orbit is no longer circular, due to the Suns smaller mass. The planets location at that moment becomes the perihelion of the new orbit, and the orbital semimajor axis increases (in this case) by a factor of 2.5.

What are the eccentricity and period of the new orbit? HINT 15. From the discussions presented in More Precisely 2-3 and More Precisely 15-1 , it may be shown that the angular momentum of a circular binary system, of separation r and component masses m1 and m2, is proportional to m1 m2 (if the total mass is constant). Such a binary has component masses one and two times the mass of the Sun, respectively, and an orbital period of two years. (a) What is its orbital separation r? (b) Mass transfer moves 0.2 solar masses of material from the more massive to the less massive star, keeping the total mass of the system fixed and conserving angular momentum. If the binary remains circular, calculate its new separation and orbital period. HINT COLLABORATIVE EXERCISES 1. Evolutionary Sequences. As a plot of luminosity vs. temperature, the HR diagram is useful for describing how stars evolve over time even though time is not the label on either axis. As a group, create an imaginary graph of dollars of financial income (vertical axis) vs.

weight (horizontal axis) and use it to describe the past and future life cycle of one of your group members. Clearly label your diagram and provide a figure caption clearly explaining each life phase. RESEARCHING ON THE WEB To complete the following exercises, go to the online Destinations Module for Chapter 20 on the Companion Website for Astronomy Today 4/e. 1. Access the "Story of a Rarely Seen Stellar Explosion" page and describe what a Sakurais object is, where it is located in the sky, and how it relates to the evolutionary future of our Sun. 2. Access the "HertzsprungRussell Diagram" page and define the four luminosity classes shown on the HR Diagram. PROJECTS 1. Can you find the Hyades cluster? It lies about 46 pc away in the constellation Taurus, making up the face of the bull. It appears to surround the very bright star Aldebaran, the Bulls eye, which makes it easy to locate in the sky. Aldebaran is a red giant, probably on the asymptotic-giant branch of its evolution.

Despite appearances, it is not part of the Hyades cluster. In fact it lies only about half as far awaysome 20 pc from Earth. 2. Now look for the Double Cluster in Perseus, h and chi Persei. These two young clusters probably formed together, and now move together through space. They lie about 2500 pc away, and are barely visible to the naked eye just east of the W of Cassiopeia. 3. Find a library that has the Astrophysical Journal. Find an article from the late 1950s and 1960s that gives the photometry of a star cluster like the Pleiades or Hyades. Plot a colormagnitude diagram (V vs. BV; see Section 17.6). Determine the V magnitude of the main-sequence turnoff, and hence estimate the age of the cluster. Compare your age with that given in the article. SKYCHART III PROJECTS The SkyChart III Student Version planetarium program on which these exercises are based is included as a separately executable program on the CD in the back of this text. 1.

An HR diagram in Figure 17.14 relates the surface temperature of a star to its luminosity. In this and the following exercise you will generate an HR diagram using spreadsheet software. SkyChart does not give us the temperature, so use the color index (see exercise 2 in Chapter 3) as its proxy. You will also need to use the absolute magnitude. SkyChart gives the apparent magnitude of stars and it can be converted to an absolute magnitude if you know the distance. Pick a region in the sky with plenty of stars (a field of about 10° should be sufficient). Select 50 stars and record their magnitude, color index (BV), and distance in pc. Some information may be missing on a particular star, so just skip it and use another. The first manipulation will be constructing a graph of distance versus apparent magnitude. Looking at your graph, can you identify a general trend? Is there more than one trend at work? Common sense tells us that stars farther away will appear less bright. In fact, the luminosity will decrease as 1/D2, where D is the distance to the star, so a star twice as far away will appear one-fourth as bright.

Does your graph support this? If it does not, can you give reasons why? What other factors besides distance play a role in a stars apparent magnitude? 2. Now generate the HR diagram. To do this, you will have to convert the apparent magnitude, m, to an absolute magnitude, M, using the following equation: M = 5-5log(D) + m, where D is the distance to the star measured in parsecs, pc. We can easily handle this equation using spreadsheet software (such as Excel). Enter in column A the name of the star, in column B the color index, in column C the distance, and in column D the magnitude. In the cell E2, enter the following expression: = 5-5*LOG10(C2) + D2. Select E2 again and select Copy from the Edit menu. Now highlight E3 through to the end of your data and select Paste from the Edit menu. You have just figured out the absolute magnitudes for all your data points.

The last calculation that needs to be done is to determine the luminosities of the stars from their absolute magnitudes according to: L = 2.512(Mo 2 M), where L is the luminosity in solar units, M is the absolute magnitude and Mo is the absolute magnitude of the Sun, which is 4.78. In cell F2, enter the equation: = 2.512^( 4.78 2 E2 ). Now Copy the formula from G2 and Paste it to cells G3 to the end of the data. Now generate a graph of luminosity versus color index, with the vertical axis on a logarithmic scale. What is the coolest star you recorded? How luminous is it? What is the least luminous star you recorded? Estimate how hot it is. What is the most luminous star you recorded? Estimate how hot it is. How does your HR diagram compare to the various HR diagrams in this chapter?

In addition to the Practice Problems and Destinations modules, the Companion Website at provides for each chapter an additional true-false, multiple choice, and labeling quiz, as well as additional annotated images, animations, and links to related Websites. What Will Happen to Earth When the Sun Dies? What Will Happen to Earth When the Sun Dies? By Jesse Emspak, Live Science Contributor | April 13, 2016 07:08am ET MORE The sun, as it appeared on April 13, 2016. Though it's been burning for some 4.5 billion years, the sun is only about halfway through its life. Credit: NASA/SDO Stars are born, they live, and they die. The sun is no different, and when it goes, the Earth goes with it. But our planet won't go quietly into the night. Rather, when the sun expands into a red giant during the throes of death, it will vaporize the Earth. Perhaps not the story you were hoping for, but there's no need to start buying star-death insurance yet. The time scale is long — 7 billion or 8 billion years from now, at least.

Humans have been around only about 40-thousandth that amount of time; if the age of the Earth were compressed into a 24-hour day, humans would occupy only the last second, at most. If contemplating stellar lifetimes does nothing else, it should underscore the existential insignificance of our lives. [ What If Earth Were Twice as Big? ] So what happens when the sun goes out? The answer has to do with how the sun shines. Stars begin their lives as big agglomerations of gas, mostly hydrogen with a dash of helium and other elements . Gas has mass, so if you put a lot of it in one place, it collapses in on itself under its own weight. That creates pressure on the interior of the proto-star, which heats up the gas until it gets so hot that the electrons get stripped off the atoms and the gas becomes charged, or ionized (a state called a plasma). The hydrogen atoms, each containing a single proton, fuse with other hydrogen atoms to become helium, which has two protons and two neutrons. The fusion releases energy in the form of light and heat, which creates outward pressure, and stops the gas from collapsing any further.

A star is born (with apologies to Barbra Streisand). There's enough hydrogen to keep this process going for billions of years. But eventually, almost all of the hydrogen in the sun's core will have fused into helium. At that point, the sun won't be able to generate as much energy, and will start to collapse under its own weight. That weight can't generate enough pressure to fuse the helium as it did with the hydrogen at the beginning of the star's life. But what hydrogen is left on the core's surface wil fuse, generating a little additional energy and allowing the sun to keep shining. That helium core, though, will start to collapse in on itself. When it does, it releases energy, though not through fusion. Instead it just heats up because of increased pressure (compressing any gas increases its temperature). That release of energy results in more light and heat, making the sun even brighter. On a darker note, however, the energy also causes the sun to bloat into a red giant. Red giants are red because their surface temperatures are lower than stars like the sun. Even so, they are much bigger than their hotter counterparts. In this artist's conception, an expanding red giant prepares to swallow a too-close gas giant planet.

In the solar system, when the sun becomes a bloated red giant, it will engulf Mercury and Venus, and may devour Earth. Credit: James Gitlin/STScI AVL A 2008 study by astronomers Klaus-Peter Schröder and Robert Connon Smith estimated that the sun will get so large that its outermost surface layers will reach about 108 million miles (about 170 million kilometers) out, absorbing the planets Mercury, Venus and Earth. The whole process of turning into a red giant will take about 5 million years, a relative blip in the sun's lifetime. [ 50 Interesting Facts About Earth ] On the bright side, the sun's luminosity is increasing by a factor of about 10 percent every billion years. The habitable zone, where liquid water can exist on a planet's surface, right now is between about 0.95 and 1.37 times the radius of the Earth's orbit (otherwise known as astronomical units, or AU). That zone will continue to move outward. By the time the sun gets ready to become a red giant, Mars will have been inside the zone for quite some time.

Meanwhile, Earth will be baking and turning into a steam bath of a planet, with its oceans evaporating and breaking down into hydrogen and oxygen. As the water gets broken down, the hydrogen will escape to space and the oxygen will react with surface rocks. Nitrogen and carbon dioxide will probably become the major components of the atmosphere — rather like Venus is today, though it's far from clear whether the Earth's atmosphere will ever get so thick. Some of that answer depends on how much volcanism is still going on and how fast plate tectonics winds down. Our descendants will, one hopes, have opted to go to Mars by then — or even farther out in the solar system. [ What If Every Volcano on Earth Erupted at Once? ] But even Mars won't last as a habitable planet. Once the sun becomes a giant, the habitable zone will move out to between 49 and 70 astronomical units. Neptune in its current orbit would probably become too hot for life; the place to live would be Pluto and the other dwarf planets, comets and ice-rich asteroids in the Kuiper Belt. One effect Schröder and Smith note is that stars like the sun lose mass over time, primarily via the solar wind.

Planets' orbits around the sun will slowly expand. It won't happen fast enough to save the Earth, but if Neptune edges far enough out it could become a home for humans, with some terraforming. The death throes of this star have resulted in a gorgeous planetary nebula called NGC 6565. Credit: ESA/Hubble & NASA, Acknowledgement: Matej Novak Eventually, though, the hydrogen in the sun's outer core will get depleted, and the sun will start to collapse once again, triggering another cycle of fusion. For about 2 billion years the sun will fuse helium into carbon and some oxygen, but there's less energy in those reactions. Once the last bits of helium turn into heavier elements, there's no more radiant energy to keep the sun puffed up against it's own weight. The core will shrink into a white dwarf. The distended sun's outer layers are only weakly bound to the core because they are so far away from it, so when the core collapses it will leave the outer layers of its atmosphere behind. The result is a planetary nebula.

Since white dwarfs are heated by compression rather than fusion, initially they are quite hot — surface temperatures can reach 50,000 degrees Fahrenheit (nearly 28,000 degrees Celsius) — and they illuminate the slowly expanding gas in the nebula. So any alien astronomers billions of years in the future might see something like the Ring Nebula in Lyra where the sun once shone. Follow Life's Little Mysteries on Twitter @ llmysteries . We're also on Facebook & Google+ . Editor's Recommendations How old is the Sun? how old is the Sun? how old is the Sun? How old is the Sun? Actually it's middle-aged. It was formed about 4,500,000,000 (four and a half billion) years ago and we expect it to carry on pretty much as it is now for a few billion years yet - so no surprises in our lifetimes! start exploring here . We can’t do your home work for you, but if you have a question about the Sun you can always Image Credit: Casey Reed/NASA A star’s life-cycle depends on its mass.

Stars like the Sun spend most of their lives quietly eating up the hydrogen in their centre. That means their temperature and size stay very much the same for billions of years. When the Sun is about 7 billion years old it will slowly start to change and will become bigger and cooler. By the time it is 10 billion years old it will have changed into a red giant and its atmosphere will stretch out to near where the Earth is today. Content on this page requires a newer version of Adobe Flash Player. The Sun will grow old in much the same way as humans do. In a few billion years time it will be unrecognisable as the Sun we know today- a bit like the difference between an 80-year old person and a photograph of them as a baby! Under-sea coral? Enchanted castles? Space serpents? These eerie, dark, pillar-like structures are actually columns of hydrogen gas and dust where new stars are being formed. This picture of the Eagle Nebula was taken with the Hubble Space Telescope. The nebula is 7000 light years from Earth and is huge - the largest pillar is about four light years from top to bottom!

The Sun and stars don't seem to change much year after year so how can we know about their lifecycles? One method is to have computer programs to calculate how a star evolves as it uses up its energy resources. Another way is to observe many stars. We see many different kinds of stars and we assume the reason for this is that we are seeing stars at different stages of their lifecycle. We can then begin to build up an explanation of how and when they change. It's a bit like observing all the people in a town centre. We assume that they are all basically the same kind of animal (human beings) but that the main differences are caused by them being at different stages of their lifecycle (age). Fortunately, we don't have the complication of male and female stars! When and how will the Sun die? The Sun will not die in the way we know plants and animals die. But in a few billion years it will have changed so much it will not be recognisable as the Sun we know today. Calculations show that, as the hydrogen fuel in the core is used up, the outer parts of the Sun will begin to expand. The Sun will turn from a 'yellow dwarf ' into a 'red giant'.

And it really means 'giant'! It will grow so big that it will almost engulf the Earth in its orbit . The outer atmosphere will eventually be puffed out in a gentle explosion and the Sun will be at the centre of a planetary nebula - just like this. , or maybe like the one below. Can you see the old star in the middle of that nebula? IRIS Weekly Questions return to top Q1: How old is our Sun? State its approximate age, and then briefly describe the remainder of its life cycle. (Oct 21-25) Answer: The Sun is approximately 4.57 billion years old. It started out as a protostar and, over time, has matured into a main sequence star. The Sun, being about halfway through its main-sequence stage, will stay in its current state for another 4 to 5 billion years, all the while increasing in temperature, brightness, and size. Then, as the Sun runs out of fuel, it will expand into a red giant before turning into a white dwarf and slowly cooling over billions of years into a black dwarf. To learn more, read this article on physical cosmology by Meredith Bower.

Featured Student Answer: The Sun is approximately 4.5 billion years old. Currently the Sun's life cycle is in the main sequence stage of the stars life. The Sun is still halfway through the main sequence star stage; it should take approximately another 4.5 billion years to complete this stage. When a medium star like the Sun uses up some of its gases, it expands to become a red giant star. At this time, it will expand to cover the orbits of Mercury, Venus, and possibly Earth. It will remain that way for about a billion years. Then, the Sun will lose its outer shell, and the core will shrink to become a white dwarf star, a hot, small star. The white dwarf star will eventually cool and stop shining to become a black dwarf, which does not emit light anymore. -Mr. Egan's class, grade 6 return to top Q2: Why does the Sun shine? Explain how our star produces energy in the form of light and heat. (Oct 28 - Nov 1) Answer: In short, the Sun shines because it is hot. Our star is made of very hot, highly dense gas primarily comprised of hydrogen and helium.

Deep in the Sun's core, where temperatures reach 15,000,000 K (27,000,000 F), four hydrogen atoms fuse together to form one helium atom, a process called nuclear fusion. The mass of the four hydrogen atoms is greater than the mass of the helium atom that they form in the fusion process. This difference in mass is converted into energy, which the Sun radiates equally in all directions. The majority of this solar energy comes in the form of the visible light and infrared (heat) regions of the electromagnetic spectrum. Featured Student Answers: The sun shines because of the heat and light emitted during the nuclear fusion reactions that take place inside its core. There the temperature and the pressure are very high, so the atoms of hydrogen combine to create helium. The mass of a helium atom is lower than the mass of a hydrogen atom, so the difference of mass is converted into energy, according to Einstein's relation between mass and energy (E=mc2). Fusion reactions are accompanied by high emissions of energy, as there are millions of tones of hydrogen that are transformed into helium every second.

This energy causes the sun gas to glow, giving off different kinds of light, like infrared light, ultraviolet light, and visible light. -Ms. Prajea's class, grade 6 and grades 9-12 The sun shines due to its extreme temperature. It is very hot and contains much energy. The sun stays hot because it is powered by the proccess called nuclear fusion located within its core. When hydrogen and helium are fused together, the mass is changed into energy. Stars are made of clouds of dust and gas that pulls inward because of gravity. As the mixture compresses, it gets hotter and hotter until a special reaction occurs. The gas particles join and expand. The process creates energy deep in the center of the star. It is this energy that gives the star heat and light. -Ms. Florio's class, grade 7 return to top Q3: Our Sun is a G-type main-sequence yellow dwarf star. What does G-type, main-sequence, and dwarf mean, and what is the actual color of our Sun? (Nov 4 - 15) Answer: Stars are classified by their temperature and the spectrum of light that they emit, or in simple terms, their color.

Using the Morgan-Keenan system, classification is done using a scale of letters: O, B, A, F, G, K, M, L, T, and Y, with O representing the hottest stars and Y representing the coolest. The hotter stars toward the O end of the scale are more blue and white in color, whereas cooler stars toward the Y end of the scale are more red and brown. Our star, the Sun, is relatively cool with a temperature between 5,200 - 6,000 degrees Kelvin (8,900 - 10,340 degrees Fahrenheit), thus earning a classification of G and a color description of yellow. Stars can be further classified according to their luminosity, or brightness, which is done using a system of Roman numerals I through VII. Because our sun is in its main sequence stage, meaning that it is still converting hydrogen to helium through the process of nuclear fusion, it is placed in class V as a dwarf or main-sequence star. These two classification systems, temperature/color and luminosity, are often represented in a scatter plot of stars called a Hertzsprung-Russell diagram.

On the diagram below you can see the Sun plotted as a class G magnitude V main-sequence yellow dwarf star. Although our star is classified as a yellow dwarf, its actual color is white. When viewing the Sun through Earth's atmosphere, the shorter wavelengths of light (violet, blue, green) are scattered, leaving us to see only the longer yellow, orange, and red wavelengths. However, if you were to travel into space above Earth's atmosphere, then you would see all the visible wavelengths of the Sun's light that, when mixed together, are seen as white light. Below is a picture representing the actual color of our Sun. Featured Student Answers: A G-type star is a star classified with G in the Morgan-Keenan system( meaning it has an average surface temeperature of 5300K-6000K, and mass and size are quite close to those of the sun). Main-sequence is the stage in a star's life while it transforms hydrogen into helium in its core by nuclear fusion, producing and emitting energy. The star doesn't grow or become smaller, so it is in an equilibrium stage. The term "dwarf star" refers to a medium star as regarding its mass, size and luminosity.

A yellow dwarf is a main sequence star whose colour is convetionally stated as yellow, colour which in reality is yellowish-white. The Sun appeares to be yellow, sometimes orange, or red due to Earth's atmosphere (photons of the lower end of the spectrum- yellow, orange and red are less easily scattered than the ones of the higher spectrum, with short wavelenghts- blue, indigo and violet). The Sun is actually white because the light it emits is a mixture of all the colours of the rainbow. The human eye can't detect this colour even in the absence of the atmosphere, so from space we see it white as well. -Ms. Prajea's class, grades 9-12 Stars are classified depending on temperature, luminosity, and spectrum. Spectral class G means that our Sun has a temperature of 5,000 to 6,000 Kelvins and emits radiation whose electromagnetic spectrum is yellow. It also means that our Sun is in yellow dwarf stage. The main sequence is a group of stars centered around the diagonal of Hertzsprung-Russell diagram.

These stars are in constant ratio of temperature to luminosity, and these are also the ones in which fusion of hydrogen turns into helium. The dwarfs are main sequence stars, which belong to the luminosity class V. Depending on the spectral type, there are different colors of dwarfs. Our sun is a GV which means that, it is a yellow dwarf. -Ms. Kafel's class, grades 9-12 return to top Q4: Does the Sun have a uniform rotation? What is the rotation period of the Sun? For what purpose is the Carrington Rotation system used? (Nov 18 - 22) Answer: The Sun has a north and south pole and rotates on its axis just as Earth does. However, because the Sun is a fluid gaseous body, it does not rotate uniformly as does a solid body like our own rocky planet. By observing the movement of sunspots, scientists have learned that different latitudes of the Sun rotate at different speeds. The Sun rotates faster at the equator and slower at the poles. It takes the Sun only 25 days to rotate at the equator, yet it takes progressively longer, up to 35 days, to rotate at the higher polar latitudes.

This is known as differential rotation. Because the Sun has a differential rotation, scientists need a system to compare locations on the Sun over a period of time, especially for the purpose of tracking sunspots. Astronomer Richard Carrington first noted that sunspots rotate every 27.28 days. This rotation period is known as the Carrington rotation and roughly corresponds to rotation at a latitude of 26 degrees, which is the typical latitude of sunspot activity. Beginning on November 9, 1853, the Carrington rotation system numbers each rotation of the Sun with a unique number called the Carrington Rotation Number. As of November 25, 2013, the current Carrington Rotation Number is 2144. Featured Student Answer: The Sun has a north and south pole, just as Earth does, and rotates on its axis. However, unlike Earth, the entire Sun doesn't rotate at the same rate because the Sun is not solid, but is instead a giant ball of gas and plasma, so different regions of the Sun don't stick together and thus rotate at different rates. This phenomenon, called differential rotation, makes the Sun's rotation period pretty hard to establish.

The Sun rotates every 25 days at the equator, and at the poles the Sun rotates every 36 days. This is known as differential rotation. The Carrington rotation system allows us to determine an average period for the Sun's rotation. It refers to comparing the positions of certain regions of the Sun over time. By following sunspots, visibly dark spots on the surface of the Sun caused by magnetic activity, Richard Carrington calculated the rotation period of the Sun relative to the stars as 25.38 days. This means about 27.27 days as seen from Earth, since our planet orbits the Sun. -Ms. Stoica's class, grade 6 return to top Q5: The Sun is a magnetic, variable star. What does that mean? What produces the Sun's magnetic field? What is a coronal hole? (Dec 2-6) Answer: A variable star is a star that changes brightness as seen from our perspective on Earth. A magnetic star is a star that generates a magnetic field through the differential rotation and convective motions of its electrically charged plasma. Our Sun does both, hence the description magnetic, variable star.

First of all, the energy output of our Sun varies by about 0.1 percent over the duration of its solar cycle, resulting in subtle variations in its apparent magnitude (brightness). Secondly, the churning motion of the Sun's convection zone generates magnetic fields that float to the surface (photosphere) and appear as darker (cooler) sunspots, serpentine filaments, and lovely coronal loops. As we learned from question four, the Sun rotates faster at the poles and slower at the equator. Over time, this differential rotation twists the magnetic field lines into knots eventually causing them to break, resulting in violent events such as flares and coronal mass ejections. To learn more about this topic and to see some excellent images of the Sun, visit the American Museum of Natural History website on SunScapes: Our Magnetic Star. Coronal holes are cooler, low density regions of the Sun's outer atmosphere (corona), where the Sun's magnetic field opens up and allows solar wind to escape. Visibly darker than their surroundings, these features can last for a few solar rotations until the magnetic field lines shift once again.

Unlike flares and CMEs that occur more frequently during heightened solar activity, coronal holes are more commonly seen when solar activity quietens. To learn more, read this brief article by Karen C. Fox of NASA Goddard Space Flight Center, SOHO Views Large Coronal Hole Near the Sun's North Pole. Featured Student Answers: The Sun is a magnetic star, which means that it has a magnetic field. It is caused by the movement of the plasma in the interior part of the Sun, which is the result of convection. The magnetic field affects the plasma, increasing the pressure without gain in density. That magnetized region expands and reaches the photosphere, consequently creating sunspots and coronal loops. The variable star is a star that is characterized by changes in magnetic fields over the course of a solar cycle during which the magnetic poles interchange their positions. -Ms. Kafel's class, grades 9-12 Coronal holes are areas of the Sun's corona where the magnetic field presents open magnetic field lines (lines of magnetic field emerging from one region that do not return to another region, but extend into space) and solar material. In fact, highly ionized hot coronal plasma is drawn out.

Coronal holes are areas where the Sun is darker and colder and the density of plasma is lower than average because some of it flows out. -Ms. Prajea's class, grade 6 return to top Q6: How long is the solar cycle on average? Where is the Sun in its current cycle? What is the Maunder Minimum? (Dec 9-13) Answer: The solar cycle, also referred to as the sunspot cycle, is an 11-year cycle on average. The end of one cycle and the beginning of the subsequent cycle is marked by the reversal of the Sun's magnetic field. Sunspots are characterized by north and south magnetic poles. In one cycle these poles will have a north-south orientation, but in the following cycle they will have a south-north orientation. This reversal happens at the end of the cycle when solar activity is low and very few, if any, sunspots are formed. This quiet period marking the end of one cycle and the beginning of a new cycle is referred to as solar minimum.

In contrast, solar maximum occurs in the middle of the cycle when solar activity is at its peak, meaning that numerous sunspots are present on the surface of the Sun and prominences, flares, and CMEs are much more common. The Sun is currently in the period of solar maximum for solar cycle 24. Solar cycle 24 began in January 2008 and is indicating its peak in 2013. Read these three articles to learn more about solar cycle 24: Backward Sunspot , Solar Cycle 24 Begins , and Twin Peaks . Watch this NASA video to see an animation of the Sun reversing its magnetic poles. Occasionally the solar cycle deviates from its 11-year norm. From 1645 to 1715, a period of about 70 years, the Sun showed very little activity and sunspots were exceptionally rare. This period became known as the Maunder Minimum, named after astronomer Edward Maunder who studied sunspot activity. Interestingly, the Maunder Minimum coincided with the coldest part of the Little Ice Age, a time when Europe and North America experienced extremely cold winters.

NASA's Solar Radiation and Climate Experiment has made a possible correlation between low sunspot activity and colder winters on Earth. [Information adapted from Maunder Minimum and Little Ice Age articles in Wikipedia.] Featured Student Answers: The solar cycle lasts 11 years on average, but there exist shorter solar cycles, about 9 years, and longer ones, about 14 years. There are sunspots visible on the Sun almost all the time during the solar maximum. Some are very large and last several weeks. The recording of solar sunspot activity began in 1755. The 24th solar cycle is the current solar cycle and began on January 4, 2008, but there was minimal activity until early 2010. The current predicted and observed size makes this the smallest sunspot cycle since Cycle 14. The Sun went through a period of inactivity in the late 17th century. This was called the Maunder Minimum. Very few sunspots were seen on the Sun from about 1645 to 1715.

This period of solar inactivity also corresponds to a climatic period called the Little Ice Age when rivers that are normally ice-free froze and snow fields remained year-round at lower altitudes. -Ms. Prajea's class, grades 9-12 The solar cycle is a periodic change in the Sun's sunspot activity. The cause of these sunspots is that the magnetic poles of the Sun are shifting. The solar cycles have two parts: the solar maximum, and the solar minimum. A solar maximum is when sunspot activity is at its peak and the Sun's magnetic poles are reversing. The solar minimum however is the opposite when there is little or no sunspot activity. Solar cycles have an average duration of about 11 years. Our sun in its current cycle is about half way done. Currently, it is in solar maximum. However in 2013, after several months of surprising calm in solar activity during solar maximum, it was clear that the Sun's activity had flat-lined. This was very surprising since 2011 was a very active year in terms of solar activity.

This unexpected calm has lead scientists to hypothesize that 2011 was merely part one of the solar maximum peak, and the second of this double-peaked solar maximum will be in early 2014. -Mr. Egan's class, grade 7 return to top Q9: What is the South Atlantic Anomaly, and how does it relate to spacecraft and satellites? (Jan 13-17) Answer: Our planet is surrounded by layers of energetic charged particles that are captured from the solar wind and stored in what is called the Van Allen radiation belts. These radiation belts are held in place by Earth's magnetic field and are located in the inner region of Earth's magnetosphere. There is an inner belt, an outer belt, and a transient third radiation belt that was discovered recently by NASA's Van Allen Probes mission. The Van Allen belts are not symmetrically placed around our planet and range in altitudes between 125 and 800 miles. The belts dip closest to Earth's surface in a region off the coast of Brazil in South America termed the South Atlantic Anomaly.

Here, in this low lying region, the International Space Station and other satellites in low Earth orbit pass through the radiation belts where for several minutes they are bombarded by the high concentration of charged particles. This can negatively affect data collection and the operation of on-board electronic systems as well as prematurely age computer and other spacecraft components. Astronauts, too, can be affected by the South Atlantic Anomaly and have reported seeing peculiar 'shooting stars' in their visual field while passing through this region. Featured Student Answers: The South Atlantic Anomaly is where the Van Allen belt is lowest and causes radiation to be the highest with satellites. -Ms. Eden's class, grades 3-5 The South Atlantic Anomaly is an area where Earth's Van Allen belt gets close to the Earth. This relates to spacecraft and satellites by increasing the levels of radiation that they detect. -Ms. Talbert's class, grade 8 The South Atlantic Anomaly is a zone where the inner radiation belt is the closest to the Earth's surface, reaching an altitude of 200 km. Here, the Earth's magnetic field is lower than its normal level.

Because of the rarefied magnetic field, the charged particles and the radiations can easily reach lower altitudes. Even though the radiation does not directly affect the Earth, it affects the satellites and the spacecraft. It interferes and causes several damages to the equipment and also affects the health of the astronauts. It harms the retina of the astronauts, and it also disables communication for a short period of time. When the spaceships pass through that zone they need more shielding, and also the telescopes cannot provide images and for their safety they enter a stand-by mode. The International Space Station requires extra shielding to deal with this problem. The Hubble Telescope does not take observations while passing through the South Atlantic Anomaly. Astronauts are also affected by this region which is said to be the cause of peculiar 'shooting stars' seen in the visual field of astronauts. The South Atlantic Anomaly is believed to be the reason for the early failures of the Globalstar network's satellites. -Ms. Stoica's class, grades 9-12 return to top Q10: The IRIS spacecraft orbits Earth in a sun-synchronous polar orbit so that it can continuously observe the Sun.

However, there are times throughout the year that IRIS is unable to see the Sun. Why do you think that is? (Jan 21-29) Answer: The IRIS spacecraft's orbit and altitude determine how long its instruments can view the Sun. The spacecraft orbits between 390 - 420 miles (620 - 670 km) above Earth's surface in what is called a sun-synchronous polar orbit. This means the spacecraft travels around Earth from north to south, passing nearly directly over the poles, in such a way that it crosses the equator at the same local time each day. Earth's axis is tilted 23-degrees, which leads to yearly changes in sun angles that coincide with the seasons. In turn, IRIS's orbit is also inclined (about 98 degrees relative to Earth's orbit around the Sun). Thus, IRIS's orbital path rides the terminator – the ever-moving line that separates the illuminated "day" side of Earth from the dark "night" side of Earth.

This path allows the spacecraft's instruments to continuously view the Sun at what we consider sunrise (dawn) or sunset (dusk) 24-hours a day for eight months during the year. However, as Earth travels along the plane of the ecliptic making its annual journey around the Sun, it will routinely eclipse the Sun, temporarily blocking the Sun from IRIS's view. As the spacecraft passes through Earth's shadow, it is unable to make solar observations. This happens during each of the spacecraft's orbits between early November and early February. These eclipse periods can last up to 24 minutes each. Read this article to learn about how the Solar Dynamics Observatory contends with this same issue twice a year: NASA's SDO Observes Earth, Lunar Transits in Same Day If IRIS were to have been launched into a slightly higher altitude orbit, it would not experience any Earth shadowing or eclipse time at all. However, these higher orbits were already too full or reserved for other spacecraft.

Featured Student Answers: IRIS travels in a polar, sun-synchronous orbit, inclined 97.89 degrees to the equator, but not at very high distances above Earth's surface ( 620-670 km) in order to avoid as much as possible Van Allen belts where the radiation is extremely high. Its orbital period is about 97 minutes. The altitude and inclination are combined in such a way that the satellite passes over any given point of the Earth's surface at the same local solar time. It travels around Earth, nearly directly over the poles, and it crosses the equator at the same local time each day. The angle between the orbital plane and the axis Earth-Sun remains almost the same. But the sun-synchronous orbits precess around the Earth's axis, and this axis is not in the plane of the ecliptic, so they don't hold a truly constant angle to the Sun. There might be periods of time, around the summer and winter solstices, when one of Earth's poles is in permanent shadow, when IRIS passes through Earth's shadow (when it passes that pole).

For maximizing the time of being in permanent sun, IRIS satellite was launched on 27 June, 2013, immediately after the summer solstice. -Ms. Prajea's class, grades 6 and 9-12 A Sun-synchronous orbit is a geocentric orbit that combines the altitude and the inclination in such a way that an object on that orbit ascends or descends over any Earth latitude at the same local mean solar time. Polar orbit satellite is the orbit that moves north-south direction in a plane perpendicular to the equatorial plane and passing through the poles. So a sun-synchronous polar orbit has an inclination close to 90 degrees and lasts 96-100 minutes. The illumination's angle surface will be nearly the same every time. Its trajectory also enables IRIS to intercept the sun's rays at roughly the same angle at all times. Theoretically, sun-synchronicity would make the sun visible the whole year round, but considering that the Earth is not perfectly spherical, it is not 100% efficient.

When IRIS will be located over the North Pole on the winter solstice, hence during polar winter, on the 22nd of December, the sun will not be visible. The same thing will repeat over the South Pole, during the northern hemisphere's summer solstice, on the 22nd of June. Also, occasionally, the moon will obstruct the spacecraft's view of the sun and since IRIS's trajectory is close to the Earth, the moon will appear large enough to block the sun as it would during an eclipse. In conclusion, the IRIS spacecraft's sun-synchronous trajectory enables it to have a clear view of the sun at most times, but certain exceptions will prevent it from being visible for small intervals. -Ms. Stoica's class, grade 6

Question: What is almost halfway through its 10-billion-year life, will expand to become a red giant and then shrink to become a white dwarf?
Answer: The sun